The present invention relates to a process for the preparation of an isocyanate by reacting a primary amine with phosgene above the boiling point of the amine over an average contact time of from 0.05 to 15 seconds under adiabatic conditions.
Various processes for the preparation of isocyanates by reacting amines with phosgene in the gas phase are known from the state of the art. EP-A-593 334 describes a process for the preparation of aromatic diisocyanates in the gas phase in which the reaction of the di amine with phosgene takes place in a tubular reactor without moving parts and with a narrowing of the walls along the longitudinal axis of the reactor. The process is problematic, however, because the mixing of the educt streams only via a narrowing of the tube wall does not function well in comparison with the use of a proper mixing element. Poor mixing usually results in an undesirably high level of solids formation.
EP-A-699 657 describes a process for the preparation of aromatic diisocyanates in the gas phase in which the reaction of the appropriate diamine with phosgene takes place in a two-zone reactor. The first zone, which makes up about 20% to 80% of the total reactor volume, has an ideal mixing system and the second zone, which makes up 80% to 20% of the total reactor volume, has piston flow. However, because at least 20% of the reaction volume is ideally back-mixed, the resulting residence time distribution is non-uniform, which can lead to an undesirably increased level of solids formation.
EP-A-289 840 describes the preparation of diisocyanates by phosgenation in the gas phase. In this disclosed process, the reaction takes place in a turbulent flow at temperatures between 200° C. and 600° C. in a cylindrical chamber without moving parts. The omission of moving parts reduces the risk of a phosgene leak.
Disregarding fluid elements in the vicinity of the wall, the turbulent flow in the cylindrical chamber (tube) achieves a relatively good equidistribution of the flow in the tube and hence a relatively narrow residence time distribution, which, as described in EP-A-570 799, can lead to a reduction in solids formation.
EP-A-570 799 discloses a process for the preparation of aromatic diisocyanates in which the reaction of the appropriate diamine with phosgene is carried out in a tubular reactor above the boiling point of the diamine over an average contact time of from 0.5 to 5 seconds. As described in the specification, both excessively long and excessively short reaction times lead to unwanted solids formation, so a process is disclosed in which the average deviation from the average contact time is less than 6%. Observation of this contact time is achieved by carrying out the reaction in a tubular flow characterized either by a Reynolds number of over 4000 or by a Bodenstein number of over 100.
EP-A-749 958 describes a process for the preparation of triisocyanates by the gas phase phosgenation of (cyclo)aliphatic triamines having three primary amine groups in which the triamine and the phosgene are reacted together continuously in a cylindrical reaction chamber heated to 200° to 600° C., with a flow velocity of at least 3 m/s.
EP-A-928 785 describes the use of microstructure mixers for the phosgenation of amines in the gas phase. A disadvantage of using such micromixers is that even the smallest amounts of solids, whose formation cannot be completely ruled out in isocyanate synthesis, can lead to clogging of the mixer, thereby reducing the time for which the phosgenation plant is available.
WO 03/045900 describes in detail the preparation of isocyanates on an industrial scale by means of gas phase phosgenation. As explained in WO 03/045900, there are two possible technical methods for carrying out the known gas phase phosgenation processes, which use a cylindrical reaction chamber. In the first method, the reaction can be carried out in a single length of tube whose diameter has to be commensurate with the production capacity of the plant. According to WO 03/045900, this design has the disadvantage, for very large production plants, that it is no longer possible to accurately control the temperature of the reaction streams in the core of the flow by heating the wall of the tube. Local temperature inhomogeneities can lead to (a) decomposition of the product if the temperature is too high or (b) inadequate conversion of the educts to the desired isocyanate if the temperature is too low.
The second possible technical method, namely division of the reaction mixture into individual partial streams that are then passed in parallel through smaller individual tubes whose temperature can be controlled better on the basis of their smaller diameter, is also regarded by WO 03/045900 as disadvantageous. According to WO 03/045900, a disadvantage of this process variant is that it is susceptible to clogging if the volumetric flow rate is not regulated through each individual tube. WO 03/045900 substantiates this by explaining that when a sediment deposits at some point in one of the tubes, the pressure loss of the flow through this tube increases and the reaction gas then automatically switches increasingly to other tubes. The consequence of this is that less gas flows through the tube containing the sediments, so the flow through the tube experiences an increased residence time, which, as already explained in EP-A-570 799, leads to an increase in solids formation.
In summary, WO 03/045900 explains that, in industrial gas phase phosgenations, the use of one large tube has the problem of temperature control of the whole flow, and the use of many small tubes runs the risk of non-uniform flow through the tubes.
According to the teaching of WO 03/045900, the disadvantages outlined can be avoided and the continuous phosgenation of amines in the gas phase can be carried out advantageously, with a substantial increase in the number of operating hours of the production plant, if the reaction is carried out in a non-cylindrical reaction channel, preferably a plate reactor, whose height preferably affords an advantageous temperature control of the reactants, and whose width is at least twice the height. As WO 03/045900 further explains, the height of the reaction channel is not generally restricted and the reaction can be carried out in a reaction channel with a height of, e.g., 40 cm. However, if a better heat exchange with the reactor walls is to be obtained, WO 03/045900 teaches that the reaction should be carried out in reaction channels of small height, e.g., only a few centimeters or millimeters, and hence with reactor dimensions at which—as WO 03/045900 indicates when commenting on EP-928 758—even the smallest amounts of solids, whose formation cannot be completely avoided in isocyanate synthesis, can lead to clogging of the reactor, thereby reducing the time for which the phosgenation plants are available.